The problem with ntp.ubuntu.com seems to be related to its load:
this morning(UTC +2), the time was not updated after 2 hours
this afternoon, the update arrives after about 10 minutes
anyway, I put now:
server 0.fr.pool.ntp.org
server 1.fr.pool.ntp.org
server 2.fr.pool.ntp.org
The problem with ntp.ubuntu.com seems to be related to its load:
this morning(UTC +2), the time was not updated after 2 hours
this afternoon, the update arrives after about 10 minutes
anyway, I put now:
server 0.fr.pool.ntp.org
server 1.fr.pool.ntp.org
server 2.fr.pool.ntp.org
which seems more reliable